This sort of moving object detection system is used as, for example, a security system or the like. For example, Japanese Patent Application Publication No. H09-272402 published on Oct. 21, 1997 discloses an ultrasonic Doppler type intrusion detection system for vehicle (hereinafter also referred to as a “first prior system”). This system detects human intrusion into a vehicle based on a phase difference between an output signal for generating ultrasonic waves and an input signal obtained from incoming ultrasonic waves. Concretely, if a Doppler shift period obtained from the phase difference is within a prescribed period range, human intrusion into a vehicle is detected.
Japanese Patent Publication No. S62-43507 published on Sep. 14, 1987 discloses a moving object detection system (hereinafter also referred to as a “second prior system”). This system is configured: to extract cosine and sine component signals from an output signal for generating ultrasonic waves and an input signal obtained from incoming ultrasonic waves; and to convert the cosine and sine component signals into binary signals to set to X and Y values of an X-Y coordinate system, respectively. An X value is 1 or 0 and a Y value is also 1 or 0, and accordingly a coordinate point (X, Y) corresponds to any quadrant of the coordinate system. The system judges that a moving object approaching the system exists in a detection area if (X, Y) turns counterclockwise around the origin of the coordinate system, and judges that a moving object leaving the system exists in a detection area if (X, Y) turns clockwise. In this prior art, existence of a moving object can be certainly detected without frequency analysis.
Japanese Patent Application Publication No. H01-189582 published on Jul. 28, 1989 discloses a moving object detection system (hereinafter also referred to as a “third prior system”). This system judges whether or not a moving object approaching or leaving an ultrasonic receiver exists in a detection area based on two Doppler signals in the same way as the second prior system. The system then increases a count value for an alarm in response to the existence of the moving object approaching the receiver, and decreases the count value in response to the existence of the moving object leaving the receiver. In this system, the influence caused by the swing of a curtain can be avoided.
In the first prior system, the human intrusion into a vehicle can be detected but it is impossible to detect whether a moving object approaches or leaves the system. In the second and third prior systems, it is possible to detect whether a moving object approaches or leaves a system (a receiver), but a moving object cannot be detected when X and Y values change within the same quadrant.